In the prior art, numerous automatic steering systems have been described that are operable to guide agricultural vehicles and implements automatically along a desired path over a field. Such automatic steering systems comprise a position sensor for detecting the actual position of the vehicle in the field, a memory storing information about a nominal, intended path of the vehicle over the field, a control unit coupled to the position sensor and to the memory and to a steering actuator that is suited to steer the vehicle. The steering actuator is controlled dependent upon the lateral offset between the actual position and the nominal path in a manner to keep the vehicle on the nominal path. A so-called feedback gain determines the relation or ratio between the lateral offset and the steering reaction of the steering actuator.
In the prior art, it was proposed to vary the feedback gain dependent on the propelling speed of the vehicle, wherein the feedback gain decreases with increasing propelling speed (U.S. Pat. Nos. 5,857,539 A, 3,946,825 A, 3,550,362 A, EP 0 381 073 A). Further, it was described to control the feedback gain dependent on the position of a moveable implement or upon a detected steering oscillation (U.S. Pat. No. 7,127,340 B), based upon a detected wheel slip rate (U.S. Pat. No. 6,901,319 B), and on vehicle kinematics (US 2004/0210357 A). It was also described to control the steering actuator based upon a detected lateral terrain slope (EP 1 688 027 A, U.S. Pat. No. 3,946,825 A).
The reaction of a moving vehicle to an action of the steering actuator depends among others on the mass or weight of the vehicle. For example, a combine with an empty grain tank follows a steering action of the steered rear wheels faster and easier than a combine with a full grain tank, since the grain mass in the grain tank corresponds to some 10% of the mass of the empty combine. This sensibility of the steering reaction to the vehicle mass is present in all work vehicles with variable payload, for example self-propelled and pulled sprayers, seeding machines, harvesting machines like combines and balers, and transport vehicles for any material. In the prior art, a variable mass or payload of a work vehicle has not been taken into account for determining the feedback gain of automatic steering systems. Dependent on the actual payload of the vehicle, the steering reaction can thus be too small or too large, leading to insufficient performance or stability of the automatic steering system.
Thus, there is a need for a simple and reliable system for controlling an automatic steering system taking the payload of the vehicle into account.
It is an object of the present invention to provide an improved automatic steering system responsive to the payload of the vehicle.